The present invention relates to wireless communications in general and more particularly to communication using multiple frequency channels, spectra or bands.
IEEE 802.11 and other contention-based Wireless Local Area Networks (WLAN) provide low-cost communication systems that help coordinate radio spectrum access for wireless communication systems according to standardized protocols. The 802.11 protocol supports data communication with moderate quality of service requirements using shared radio frequency channels. It was designed to be simple to use, and robust when operating in interference prone environments typical of unlicensed spectra. Such spectra are unlicensed in the sense that compliant devices are free to transmit without previously arranged spectrum or frequency channel assignments.
Accordingly, contention-based protocols are useful in uncoordinated environments, where distributed and independent decision-makers share resources. In wireless communications, one important resource to share by base stations and mobile stations is the available or designated radio spectrum, which is a limited resource. When accessing the available or designated radio spectrum in a distributed way, stations access resources repeatedly for a limited duration, before releasing them again for use by other stations. Transmitting data from a base station to a mobile station is referred to as downlink, whereas communication in the opposite direction is referred to as uplink. Downlink and uplink data transmission in traditional contention-based wireless communication and networking environments typically occur on the same radio spectrum, i.e., the same radio frequencies or frequency channels. Frequency channels sometimes include a block of frequencies in some continuous area of the radio frequency spectrum.
With contention-based protocols, a station waits for a contention period of random duration and, if no other stations begin sending data by the end of that the random wait time, the station will start sending data over the shared frequency channel. However, if another station starts to send data on the shared frequency channel before the end of the contention or wait period, then the station pauses its timer and waits until the spectrum resources are available again, before resuming the contention period count down. Systems that operate in this manner are often referred to as a listen-before-talk or listen-before-send type systems or protocols. Since stations in listen-before-talk systems select random waiting times independently, multiple stations may initiate data exchange at the same time, which is referred to as collision and results in lost data and the undesirable need for re-transmissions. Collision probabilities can be reduced with increasing the contention window at the potential cost of spectrum efficiency. The addition of frequency channels to the available spectrum can alleviate the congestion on heavily trafficked WLANs.
Recently, previously licensed frequency channels have been opened up and made available to unlicensed device and protocols due to changes in certain regulations. For example, TV white space (“TVWS”) refers to spectra that were previously licensed to analog television broadcasters, but is now unlicensed following the transition of licensed television broadcasts from analog form to digital form in different frequency ranges. While the use of TVWS will be unlicensed, there are certain requirements for transmitters and receivers in the allocated wireless frequency ranges that are designed to avoid widespread interference in those frequency ranges.
To operate in the newly released TVWS, a TVWS device (TVWD) should be able to detect unused radio frequency channels in which to operate. Channel availability may change dynamically depending on the environment and the original spectrum usage by the primary system. Some rules of using TVWS prohibit emitting radio signals in certain channels, to avoid unwanted interference to primary radio systems such as terrestrial TV receivers that may be located in close proximity. However, the rules regarding listening in a TVWD (i.e., listen, measure signals, or receive data) are more relaxed relative to the rules regarding transmitting, because listening does not normally create interference.
In the United States, the Federal Communications Commission (“FCC”) regulates wireless transmissions and other countries have radio regulators as well. As for TVWS, FIG. 1 illustrates the FCC's TVWS regulation. Solid lines indicate permissions for secondary usage of TV channels for fixed or portable TV white space devices (TVWDs). Dotted lines indicate channels that are excluded from TVWS. After the digital transition, channels 52-69 will be used for next generation cellular and for public safety applications. The FCC, in its “Unlicensed Operation in the TV Broadcast Bands” report, describes the regulatory framework for the TVWS. The FCC defines two classes of TVWDs: (1) fixed TVWDs and (2) personal/portable TVWDs. The personal/portable TVWDs operate either under control of a fixed TVWD or autonomously. Different regulatory rules are defined for the two different device classes.
Fixed TVWDs are permitted to operate in the VHF channels except channels 3-4, and on the UHF channels, except channels 36-38. The reason for the exclusion of channels 3-4 is to prevent interference with external devices (e.g., DVD players) that are often connected with shielded cables to a TV utilizing these channels. The exclusion for channel 36-38 is to prevent interference with radio astronomy measurements at channel 37. In FIG. 1, exclusions are indicated by dotted instead of solid lines for fixed and portable services.
The operation of portable TVWDs is even more restricted, because of their potentially nomadic mobility pattern. Portable TVWDs are only permitted to operate in the UHF channels starting from channel 21, and with the exception of channel 37. Portable devices are not permitted on channels 14-20, since some metropolitan areas of the U.S. use some of those channels for public safety applications.
For the different VHF and UHF channels, the proposed regulation defines maximum power levels for the fixed and for the portable TVWDs. Fixed TVWDs are permitted to transmit with up to 30 dBm (1000 mW) and additionally with up to 6 dBi antenna gain, resulting in maximum 4000 mW EIRP. Portable TVWDs are permitted to transmit with up to 20 dBm (100 mW) with no antenna gain. A contour protection requires portable TVWDs to further reduce the transmission power when operating on channels adjacent to active TV broadcast channels. The capability to dynamically reduce the transmission power when possible is required for all TVWDs. In general, the requirements of fixed TVWD devices are more relaxed relative to portable TVWDs. All TVWDs must be able to sense spectrum and identify analogue and digital TV signals (ATSC and NTSC TV signals), plus auxiliary primary signals such as wireless microphones, all at a level of −114 dBm (averaged over a predefined bandwidth).
A central geo-location database controls mechanism of TVWS operations. Permission to access channels is given in a location grid of 50 meter accuracy. All fixed TVWDs must provide accurate geo-location at 50 meter accuracy and database access, as well as for all autonomous portable TVWDs that are not operating under control of a fixed TV WD. In the database, an entire venue such as an entertainment venue or an event can be registered and protected against any secondary TVWS activity. Instead of relying on geo-location with GPS or similar means, for fixed TVWDs, a certified installer can configure the device location during setup. Further, fixed TVWDs must transmit their identities and location information to facilitate their detection in interference scenarios.
FIG. 1 illustrates allocation of regulated channels for 802.11 frequency division multiplexing. This is particularly useful where regulations or technical considerations have distinct rules for fixed and portable channels. In such cases, if the more constraining rules apply to one set of frequencies, such as allowing only transmissions from fixed devices, a downlink from a fixed device might be on that one set of frequencies, whereas the uplink is on a more relaxed set of frequencies.
FIG. 2 illustrates the application of TVWS regulations for three example locations in the U.S. In densely populated areas, such as Market Street in San Francisco (top line) or Wall Street in New York City (bottom line), the number of available TVWS channels are zero or close to zero. In Orlando, Fla., a few free channels are available in UHF and in VHF, but because of the restrictive regulation, none of them would be available for fixed and portable secondary spectrum usage with TVWDs. FIG. 2 also illustrates allocation of channels for frequency division multiplexing using portable devices. In some cases, FDM helps with TVWS operations. In the example shown in FIG. 2, TV channel 9 is used for fixed/downlink traffic and TV channel 28 is used for portable/uplink traffic.
Television broadcast signals are protected with a contour, inside which TV white space devices are not allowed to transmit. Within the protected area defined by the contour, there are special rules for operation on a TV channel adjacent to an active TV broadcast channel. Fixed TV white space devices are not permitted to operate on channels adjacent to TV broadcast channels that are allocated to TV broadcast services. Portable devices are permitted to operate on an adjacent TV channel with the maximum allowed transmission power of 16 dBm (4 dB lower than on non-adjacent channels, i.e., 40 mW).
The release of TVWS is one of the biggest allocations of spectrum for unlicensed use that has been created in recent history. Embodiments of the present invention take advantage of the newly available spectrum to increase the efficiency with which bandwidth in WLANs can be utilized.